1. Field of the Invention
The present invention relates to a device and method for optically detecting a surface defect of a round wire rod in real-time and without contact. The device and method can detect the surface defect using an optical sensor, in which the surface defect occurs on the wire rod in a process of manufacturing the wire rod through rolling, drawing and extrusion.
2. Description of Related Art
Technologies for detecting a surface defect of round wire rods include ultrasonic test, Magnetic Flux Leakage (MFL), Magnetic Particle Inspection (MPI), eddy current inspection, optical inspection and so on.
The ultrasonic test is a method that determines whether or not a bar or wire rod has a surface defect by transmitting a surface ultrasonic wave from an ultrasonic generator to the wire rod subjected to surface defect detection, receives an ultrasonic wave signal reflected from the wire rod, and analyzes the reflecting ultrasonic wave signal. The ultrasonic test is excellent for detecting non-continuous surface defects such as a crack formed in the direction perpendicular to the transmission direction of the ultrasonic wave. However, it is not effective for detecting some surface defects, which are formed along the transmission direction of the ultrasonic wave or are followed by a smooth shape change. The ultrasonic test can hardly transmit ultrasonic energy to the whole surface of a circular object to be tested (hereinafter, referred to as “test object”). In particular, the efficiency of transmitting an ultrasonic wave from the ultrasonic generator to the test object is poor when the test object has a large surface roughness, is accompanied with vibration, is hot, or is being transmitted.
The magnetic flux leakage (MFL) can excellently detect cracks in or under the surface of ferromagnetic metal. The detection performance of the MFL is excellent for fine cracks even if surface roughness is very large, but is not effective for cracks formed along the direction of a magnetic flux, which is generated on the surface of a ferromagnetic object, or when a defect has a smooth edge.
The principle of the MFL is as follows: When an air gap is formed in a crack of a test object or impurities are accumulated in the crack, permeability characteristics become different from those of a ferromagnetic body. While the magnetic flux is continuously formed to be parallel to the surface of the test object when the surface has normal conditions, a magnetic flux leaks in the direction perpendicular to the surface when there is a permeability difference. Then, a defect such as a crack is detected by measuring a magnetic flux leakage using a magnetic flux leakage sensor.
The MFL may cause some problems when applied to a round bar that is hot and is transported at a high speed.
Firstly, since the magnetic flux leakage sensor is not stable with temperature changes, the stability of the magnetic flux leakage sensor is not maintained when applied to a hot material.
Secondly, it is very difficult to generate a magnetic flux on the surface of a predetermined portion of a steel member that is moving at a high speed.
Thirdly, a magnetic flux leakage signal detected by the magnetic flux leakage sensor is inverse proportional to square of distance. When a wire rod, which is being transported at a high speed, vibrates, a pseudo defect occurs since it is difficult to discriminate a defect signal from a vibration signal.
Fourthly, a constant distance has to be maintained between the surface of a test object and the sensor in order to detect a defect of a round bar. It is also required to arrange a sensor system in circle. Here, a sensor head has to be replaced whenever the diameter of the round wire bar changes. Since products having a variety of diameters are manufactured on a single manufacturing line, an operation of replacing the sensor head in the manufacturing line according to changes in diameter causes a considerably large amount of load.
The Magnetic Particle Inspection (MPI) is very similar to the MFL in a process of forming a magnetic flux leakage when a test object has a defect in the surface. The MPI directly measures the magnetic flux leakage formed in the defect of the test object using a sensor capable of detecting the magnetic flux leakage and distributes magnetic particles coated with fluorescent material on the test object in order to more clearly clarify information on the formed magnetic flux leakage. An area of the test object having the magnetic flux leakage attracts the magnetic particles using a magnetic attractive force, but a normal area of the test object does not attract the magnetic particles. Since the fluorescent material, which sensitively reacts with ultraviolet rays, is coated on the magnetic particles for a visual effect, it is possible to acquire the geometry of the defect by emitting ultraviolet light. Unlike the ultrasonic test or the MFL, the MPI can acquire the distribution of magnetic particles corresponding to the geometry of the defect so as to classify the defect based on its geometry information. Since the MPI detects a defect using an optical sensor instead of the magnetic flux leakage sensor, it can overcome some drawbacks of the MFL related with vibration or the magnetic flux leakage sensor and thus is widely used. However, the MPI is generally used when the test object has a temperature 70° or less due to limited temperature characteristics of the fluorescent magnetic particles. Since the MPI requires an additional work such as magnetic particle inspection and forming of a magnetic field on the test object, it is difficult to apply the MPI to a continuous manufacturing line such as a rolling line.
The eddy current inspection is a technique using electromagnetic characteristics of metal, and is applicable to a material such as a hot bar, which is continuously manufactured using an eddy current sensor having a relatively short response time. The eddy current inspection has a drawback in that defects may increase when the test object vibrates since the eddy current sensor has to be arranged very close to the test object as in the sensor arrangement of the MFL. Since a defect of the signal having a predetermined threshold or more is qualitatively determined by analyzing an analog signal generated by the eddy current sensor, it is difficult to make a quantitative determination on for example the size, length and height of the defect. In particular, some defects having a specific geometry are not easily detectable. In general, the eddy current inspection is widely used to statistically analyze overall changes in test objects according to changes in manufacturing conditions or times rather than detecting respective defects and evaluating characteristics.
The optical inspection is generally divided into two methods. The first method is to discriminate a defective portion from a normal portion by directly receiving light, which is spontaneously emitted from a hot test object. The second method is to discriminate a defective portion from a normal portion by emitting light from an external light source to a hot test object and receiving light reflected from the test object.
1. First Method
Referring to FIG. 1, light energy radiated from the surface of a hot round wire rod 2 is received using an optical sensor 1 and a defect is detected by discriminating between a sensor signal from a normal portion and a sensor signal from an abnormal portion.
Emissivity is the level of energy radiated outwards from the surface of a material. The emissivity of hot metal is varied according to the temperature, surface characteristics and types of metal. When metal has a surface defect, its emissivity is varied owing to the difference in roughness, area and surface luminance between a defective portion and a normal portion. The varied emissivity causes the defective portion to emit a different amount of energy from the normal portion. In order to observe the defective portion of the metal surface, on the assumption that the temperature of metal is constant and the construction of the optical sensor is uniform, the difference in emissivity between the defective and normal portions changes the characteristics of light emitted from the round wire rod 2 and influences the output voltage of the optical sensor 1. In particular, factors influencing emissivity include the difference in surface roughness between the defective and normal portions, the difference in components between the defective and normal portions and the difference in temperature between the defective and normal portions.
In the case of using light spontaneously emitted from the round wire rod 2, when the difference in surface luminance between the normal and defective portions is great, the difference in emissivity between the normal and defective portions increases. As excellent characteristics, the power of discrimination of defects can be raised by increasing the difference between the response values of the optical sensor 1. When the emissivity difference between the defective and normal portions is not large or the emissivity of the defective portion does not have predetermined characteristics, the detection method using spontaneous emission is not effective.
2. Second Method
Referring to FIG. 2, the optical inspection method uses light emitted from an external lighting device 3. The external lighting device 3 emits light having wavelength characteristics different from or the same as those emitted from a round wire rod 2. The intensity of light emitted from the lighting device 3 is set to be greater than that emitted from the round wire rod 2.
When light emitted from the lighting device 3 has a wavelength band different from that emitted from the hot round wire rod 2, there is required an optical filter 5 that transmits the light emitted from the lighting device 3 but does not transmit the light emitted from the hot round wire rod 2. The optical filter 5 used can shield radiation energy emitted from the hot round rod 2 and minimize an influence of light emitted from the hot round rod 2 on the optical sensor 1. When the sensitivity of the optical sensor 1 is poor, the intensity of light emitted from the lighting device 3 has to be increased. The arrangement of the lighting device 3 has to be designed according to the surface geometry of the round wire rod 2.
As shown in FIG. 3, when the test object is shaped like a slab, the external lighting device 3 can emit light so as to be uniformly reflected from the surface of the slab 6. This provides uniform signal characteristics to an optical sensor 1 (also referred to as signal detecting sensor) so that defects can be detected from a wide surface area. It can be appreciated that the sensor signals having a substantially uniform magnitude are obtained from the central portion and edge portions along the width direction of the slab 6.
As shown in FIG. 4, in the case where a test object is a round wire rod 2, when external light is emitted from an external lighting device 3, a reflection angle at a reflection point is the same as an incident angle with respect to a normal from the surface. As a result, defect inspection can be performed on only a small area since only a small surface area can reflect light toward an optical sensor 1 (also referred to as signal detecting sensor).